This autoimmune disease of muscle weakness is caused by destruction of acetylcholine receptors.

Correct Answer: A

Rationale: Myasthenia gravis, an autoimmune condition, weakens muscles by antibodies destroying acetylcholine receptors, impairing nerve-muscle signals, causing fatigue. Amyotrophic lateral sclerosis (ALS, Lou Gehrig's) is neurodegenerative, not autoimmune. Fibromyalgia is pain-focused, not receptor-based. Myasthenia's receptor-specific weakness sets it apart, treatable with immunosuppressants, distinct from nerve or pain conditions.

Correct Answer: D

Rationale: Adipose tissue is the body's largest endogenous substrate source, storing vast triglyceride reserves thousands of grams far exceeding other stores, providing energy via lipolysis during prolonged activity. Muscle glycogen, about 300-500 grams in trained individuals, fuels local contraction but is limited. Liver glycogen, around 100 grams, maintains blood glucose but depletes quickly. Intramuscular lipids, small triglyceride droplets, contribute modestly, far less than adipose's capacity. Adipose tissue's sheer volume and energy density (9 kcal/g) dwarf glycogen's stores (4 kcal/g), distinguishing it as the primary reserve, key for sustained energy needs beyond glycogen's scope.

Correct Answer: C

Rationale: Pectoralis major has a convergent fascicle pattern, with fibers fanning from a broad origin (clavicle, sternum) to a narrow insertion (humerus), enhancing force for arm adduction and rotation. Biceps brachii has parallel fascicles along its length for elbow flexion. Gluteus maximus features multipennate fibers for hip extension. Rectus femoris runs parallel for knee extension. Convergent patterns concentrate force from multiple directions, distinguishing pectoralis major's structure and function, key for chest movements, unlike parallel or pennate arrangements.

Correct Answer: C

Rationale: Erector spinae, along the spine, extends and stabilizes it, providing postural support for standing and lifting. Arm movement involves deltoid or biceps. Pelvic stabilization uses gluteals or obliques. Rotation involves rotatores or obliques, not erector spinae's primary extension. Its role in maintaining upright posture distinguishes it, essential for spinal integrity, unlike arm, pelvic, or twisting functions.

Correct Answer: B

Rationale: Vastus intermedius, a quadriceps muscle, lies deep to rectus femoris, which overlays it on the anterior thigh, both extending the knee. Biceps femoris, a hamstring, is posterior. Vastus medialis and lateralis flank it laterally and medially, not superiorly. Rectus femoris' superficial position distinguishes it, key for quadriceps layering.